1. Field of the Invention
This invention pertains generally to the field of aircraft display units that provide flight information to the pilot or flight crew of an aircraft.
2. Description of the Related Art
The generation of a three-dimensional image of terrain in a synthetic vision system may be accomplished using OpenGL, a standard specification defining a software interface to graphics hardware. A series of processing steps known generally as an “OpenGL Pipeline” are employed to render three-dimensional image data for presentation on the two-dimensional screen of a display unit. One of the processing steps known generally as “OpenGL Transformation” receives vertex data such as three-dimensional terrain data and transforms such data into two-dimensional screen coordinates (or window coordinates) data.
OpenGL Transformation is a series of processing steps known to those skilled in the art, and only a brief summary of these steps is provided. Coordinates of an object may be derived from vertex data, where object coordinates are measured in reference to object space. When the object coordinates are multiplied by a matrix known generally as GL_MODELVIEW, eye coordinates of the object are yielded, where eye coordinates are measured in reference to eye space. When the eye coordinates are multiplied by a matrix known generally as GL_PROJECTION which integrates, clipping and normalized device coordinates (“NDC”) transformations, the eye coordinates of the object will become mapped to NDC. Then, by means of a viewport transformation, the coordinates of the NDC are scaled and translated in order to fit into the screen.
The GL_PROJECTION matrix includes a defined frustum for the purpose of performing the clipping transformation. In a perspective projection, a pyramid frustum originating from the origin of the eye coordinate system is generally employed, where the pyramid frustum has been truncated by a near plane (or projection plane). Objects and partial objects falling inside the truncated frustum will be projected onto the near plane, and those falling outside of the frustum will be clipped out.
A notable feature of the projection is that identical objects located at different positions within the frustum and at the same distance from the origin of the eye coordinate system may appear differently on the near plane as a result of the projection. An object located near the periphery of the frustum will appear to be stretched and distorted on the near plane from that of an identical object located near the center of the frustum. For example, a sphere located near the periphery of the frustum will appear to be stretched and distorted when compared with the projection an identical sphere located near the center of the frustum and at the same distance from the origin. Without correction, this stretching and distortion survives NDC and viewport transformations.
When applied to the generation of a three-dimensional terrain image, terrain located at the same distance from the aircraft position will appear to stretch outwardly from the center of the image when presented on the two-dimensional screen, where the center of the image is based on the center of frustum. If symbology representative of attitude (e.g., pitch scale or pitch tape) provides for a linear scale with equally-spaced graduations, a non-conformality may result between the symbology and the image of terrain when the former is presented against the background of the latter.